Error due to time management

Given csv data that looks like this:
I want to save the low/high at 9:31(the earliest time for each day) and use it to compare against current values. This is what my implementation looks like right now:
I get this error, but I can't figure out exactly what is causing it:
Is there anything glaringly wrong with what I did? What do I need to do to get the desired effect? Thanks.